A method for preparing a lithographic printing plate by an electrophotographic method is known. In general, the lithographic printing plate is prepared by the successive steps of uniform charging of a photoconductive layer of an electrophotographic plate-making material, obtaining a toner image by a wet or dry development after imagewise exposure, and making non-image areas which are free of toner hydrophilic by a treatment with a desensitizing solution (etching liquid) after the toner image is fixed.
As such electrophotographic plate-making materials, ones comprising a paper support are known. However, lithographic printing plates obtained from such materials possess a press life of only around 3,000 sheets. The main reason for this short press life is permeation of water into the paper support. Specifically, permeation of etching liquid (which is an aqueous solution) used in the desensitizing process at non-image areas after plate-making and permeation of dampening water used during printing occur to cause stretching of the paper support due to water absorption. In an extreme case, exfoliation between the paper support and the photoconductive layer occurs.
With respect to print quality, for example, about 100 lines/inch is the limit of dots reproducebility.
Further, the water content of the support varies according to the temperature and humidity conditions at exposure; as a result, conductivity changes, which affects electrophotographic performance.
Various proposals have been made to overcome the above problems. One employs an intermediate layer between the base paper and the photoconductive layer. For example, the following have been proposed: the use of an intermediate epoxy resin layer in Japanese Patent Application (OPI) No. 138904/75 (the term "OPI" as used herein refers to a "published unexamined Japanese Patent Application"); the use of an intermediate ethylene derivative layer such as an ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ethylene-vinyl acetate copolymer, ethylene-vinyl acetate-vinyl chloride copolymer, ethylene ionomer and the like in Japanese Patent Application (OPI) No. 105580/80; the use of an intermediate layer formed by coating with an aqueous polyethylene emulsion mixed with carbon black or graphite and drying in Japanese Patent Application (OPI) No. 14804/79.
However, the use of electrophotographic plate-making materials having the above intermediate layers still does not provide a lithographic printing plate with a long press life.